Most electricity used in the United States comes from power grids fed by large power stations. However, for many reasons alternate energy systems are becoming economically attractive in special situations. These alternate energy systems include solar, wind, hydroelectric and thermoelectric generators. Solar electric generators (SG's) have been commercially available in the United States for about 25 years. These units generate electric power from the energy of sunlight, which is free. Attempts have been made to produce electric power from sunlight to supply utility electric grids but these efforts have been largely unsuccessful because the total cost per kilowatt-hour from the solar generators substantially exceed the cost per kilowatt hour for electric power generated at central generating stations powered by burning coal, oil, gas or by nuclear power plants.